1. Field of the Invention
The present invention relates to a bootstrap circuit and a method for driving the bootstrap circuit, and also to a shift register circuit, a logical operation circuit, and a semiconductor device each using the bootstrap circuit. More particularly, the present invention relates to a semiconductor device including a circuit that outputs digital pulses, such as a shift register for driving a display device, a camera and the like and an address decoder for driving a memory device.
2. Description of the Related Art
For a circuit constituting a semiconductor device, in many cases, a CMOS (Complementary Metal-Oxide Semiconductor) circuit using an N-channel MOS (NMOS) transistor and a P-channel MOS (PMOS) transistor is generally employed because of its low power consumption. In a CMOS circuit, when digital pulses are outputted, high and low levels of potential can be outputted by using high potential (VH) and low potential (VL) as power source. In other words, the CMOS circuit can be configured such that the PMOS transistor is made conducting to output the high potential and that the NMOS transistor is made conducting to output the low potential. However, the manufacture of a CMOS semiconductor device requires multiple processes of impurity implantation to form PMOS and NMOS, in addition to other processes such as film formation, mask exposure, and etching. Accordingly, there arises a problem of an increase in cost.
On the other hand, in the case of a semiconductor device composed of MOS transistors of a single conductivity type (P-type or N-type) only, it is possible to reduce the number of processes for impurity implantation and the like in its manufacturing processes, and therefore the manufacturing costs can be reduced. However, because of a single conductivity type of semiconductor, there arise problems that the power consumption increases and the output margin decreases as compared with the CMOS transistor. More specifically, in the case of a circuit composed of PMOS transistors only, when a low potential is outputted, its output voltage becomes higher than the low potential by a potential corresponding to the threshold voltage of the transistor. In the case of a circuit composed of NMOS transistors only, when a high potential is outputted, its output voltage becomes lower than the high potential by a potential corresponding to the threshold voltage of the transistor.
In order to solve the above-mentioned problems, dynamic circuits using bootstrap effect have been proposed and utilized. An example of a general bootstrap circuit is shown in FIG. 1A, which is described in Mohamed I. Elmasry, “Digital MOS Integrated Circuits,” IEEE PRESS, 1981, p. 48. This circuit includes a PMOS transistor 101 that outputs pulses from its source, a PMOS transistor 102 connecting the gate electrode of the transistor 101 and a power source that outputs a low potential VL, and a coupling capacitor 103 connected between the source and gate electrodes of the transistor 101.
A pulse signal S1 to be inputted to the drain electrode of the transistor 101 and a pulse signal S2 to be inputted to the gate electrode of the transistor 102 have two potential levels, low VL and high VH.
An operation of the above circuit will be described hereinafter. As shown in the timing chart of FIG. 1B, first, in a period A, when the pulse signal S2 becomes at the low potential VL, the potential of a node N1 decreases to a potential VL′ that is higher than VL by a potential corresponding to a threshold voltage Vth of the transistor 102.
Here, a threshold voltage is defined as a voltage between the gate and source electrodes of a transistor when current flowing between the source and drain electrodes of the transistor becomes 10 nanoamperes. In the case of a PMOS transistor, it is assumed that the transistor is made conducting when the gate-source voltage is smaller than the threshold voltage. Accordingly, when the pulse signal S1 is at the high potential VH, the transistor 101 is brought into conduction, and the potential of an output OUT, which is the source of the transistor 101, becomes high (VH). The coupling capacitor 103 is charged with a voltage of (VH−VL′).
Next, in a period B, when the pulse signal S2 rises to the high potential VH, the transistor 102 is brought out of conduction, and the node N1 is brought into a floating state. When the potential of the pulse signal S1 falls from VH to VL, the potential of the output OUT also decreases from VH to VL because the transistor 101 is in the conductive state. At this time, since the output OUT and the node N1 are coupled by the capacitor 103, the potential of the node N1 shifts toward lower potential. Since the potential of the node N1 drops below the low potential VL, it is possible to output the low potential VL from the output OUT, with the transistor 101 kept in the conductive state. If the charges stored in the coupling capacitor 103 are not redistributed to any other capacitor, then the potential of the node N1 drops down to (VL+VL′−VH).
A conventional circuit as shown in FIG. 1C has also been used as a circuit that produces a similar bootstrap effect. This circuit differs from the circuit of FIG. 1A in that the transistor 102 is diode-connected. Although the bootstrap circuit includes only PMOS transistors in this example, it is also possible to similarly construct a bootstrap circuit by using only NMOS transistors. An example of such a circuit is shown in FIG. 1D. This bootstrap circuit includes an NMOS transistor 104 that outputs pulses from its source, an NMOS transistor 105 connecting the gate electrode of the transistor 104 and a power source that outputs a high potential VH, and a coupling capacitor 106 provided between the source and gate electrodes of the transistor 104.
A pulse signal S1 to be inputted to the drain electrode of the transistor 104 and a pulse signal S2 to be inputted to the gate electrode of the transistor 105 have two potential levels, low VL and high VH. An operation of this circuit will be described hereinafter.
As shown in the timing chart of FIG. 1E, first, in a period A, when the pulse signal S2 becomes at the high potential VH, the potential of a node N1 increases to a potential VH′ that is lower than VH by a potential corresponding to a threshold voltage Vth of the transistor 105.
Here, in the case of an NMOS transistor, it is assumed that the transistor is made conducting when the gate-source voltage is greater than the threshold voltage. At this time, when the pulse signal S1 is at the low potential VL, the transistor 104 is brought into a conductive state, and the potential of an output OUT, which is the source of the transistor 104, becomes VL. The coupling capacitor 106 is charged with a voltage of (VH′−VL).
Next, in a period B, when the pulse signal S2 falls to the low potential VL, the transistor 105 is brought out of a conductive state, and the node N1 is brought into a floating state. When the potential of the pulse signal S1 rises from VL to VH, the potential of the output OUT also increases from VL to VH because the transistor 104 is in the conductive state. At this time, since the output OUT and the node N1 are coupled by the capacitor 106, the potential of the node N1 shifts toward higher potential. Since the potential of the node N1 rises above the high potential VH, it is possible to output the high potential VH from the output OUT, with the transistor 104 kept in the conductive state. If the charges stored in the coupling capacitor 106 are not redistributed to any other capacitor, the potential of the node N1 rises up to (VH+VH′−VL).
In the bootstrap circuit composed of PMOS transistors shown in FIG. 1A, the potential of the node N1, when dropping below the low potential VL due to the bootstrap effect, depends on the initial potential VL′ before the bootstrap effect occurs. When the potential VL is applied to each of the gate and drain electrodes of the transistor 102, the potential of the node N1, which is the source electrode of the transistor 102, stabilizes at VL′=VL−Vth. Here, Vth is the threshold voltage of the transistor 102. In other words, the initial potential VL′ depends on the threshold voltage of the transistor, and therefore a potential drop due to the bootstrap effect also varies with the characteristics of the transistor.
A similar phenomenon also occurs in the case of NMOS transistors, which is described in detail in Japanese Patent No. 3422921. According to the specification of this patent, when the potential VH is applied to the drain electrode of the transistor 105 in FIG. 1D, the potential of the source electrode stabilizes at a potential lower than the potential VH by the threshold voltage Vth of the transistor 105.
In conventional bootstrap circuits, if there are variations in the threshold voltages of transistors, variations in the initial potential VL′ are caused, leading to variations in bootstrap-dropped voltage in the same range. For example, it is assumed that manufactured transistors have variations in threshold voltage, ranging from a maximum threshold voltage Vthmax to a minimum threshold voltage Vthmin. The initial potential VL′, which is the potential of the node N1 during the period A in FIG. 1B, is (VL−Vthmin) at the maximum and (VL−Vthmax) at the minimum, varying in a range equivalent to that of the variations in threshold voltage. Therefore, the potential of the node N1 that drops in the period B is (2VL−Vthmin−VH) at the maximum and (2VL−Vthmax−VH) at the minimum.
When the potential of the node N1 drops to (2VL−Vthmax−VH) in the period B, the largest voltage difference that could be produced during the operation of the bootstrap circuit increases up to (2VH+Vthmax−2VL), which is the difference between the maximum voltage VH and the minimum voltage. That is, in the case where Vthmax is large and hence variations in threshold voltage are wide, the demand for the withstand voltage performance of the transistors is increased.
On the other hand, when the potential of the node N1 drops to (2VL−Vthmin−VH) in the period B, the voltage difference between the gate and source electrodes of the transistor 101 in the conductive state becomes small. That is, in the case where Vthmin is small and hence variations in threshold voltage are wide, there is a possibility that the conduction characteristic may become insufficient. The same problem also arises in the case of NMOS transistors.
A shift register with a bootstrap circuit as described above being applied to an output section is described in Japanese Patent Application Unexamined Publication No. 2002-215118. As described therein, in the case of applying the shift register to a scanning line drive circuit of a display device, the drive circuit is composed of a shift register including several hundreds of stages corresponding to the resolution of a screen. Therefore, if the transistors constituting the first stage to the final stage of the shift register vary in threshold voltage, the demand regarding the transistor withstand voltage is increased, and the differences in the conduction characteristic may cause deterioration in a display image.
Additionally, if pixel transistors and the scanning line drive circuit are formed of single-conductivity-type thin film transistors simultaneously on a substrate of the display device by using a thin film transistor (TFT) technique, advantages are obtained such as a reduction in the manufacturing costs and the improved reliability of scanning wiring connections. However, higher withstand voltage properties are demanded of the transistors. This is because thin film transistors generally have high threshold voltages and have wide manufacturing variations in threshold voltage, as compared with transistors manufactured by a single-crystal semiconductor technique.
Accordingly, a phenomenon to be avoided is that a bootstrap effect causes a change in potential. In other words, a problem to be solved is that variations occur in the potential that drops or rises due to the bootstrap effect. When a potential change is large, a high voltage is applied between the electrodes of the transistor, causing deterioration. When a potential change is small, the transistor is brought into insufficient conduction, which may cause a trouble in the operation of the circuit, resulting in degraded reliability of a semiconductor device.
The reason why such a problem occurs is that an initial voltage applied to the gate electrode of a transistor before a bootstrap effect occurs depends on the threshold voltage of another transistor that provides the initial voltage. Accordingly, variations also occur in the potential that varies in the same range as that of manufacturing variations in transistor threshold voltage.